High energy costs have resulted in concerted efforts by industry to produce goods that reduce energy waste, e.g., by producing high-mileage automobiles, more effective home insulation, and more cost-effective home appliances. Competitive market pressures now require that products be energy-efficient and that consumers be provided with numerical data by which the public may compare competing products, e.g., estimated average operating costs in "dollars per year" for home appliances such as refrigerators and freezers.
Naturally, while the public will favor energy efficient products, the manufacturing and sales price of any product must not be unreasonably high. The focus, therefore, has to be on how to manufacture products at the lowest possible cost and with good performance characteristics. In the manufacture of refrigerators and freezers, consequently, improvements are sought for low cost, highly effective, easy-to-handle insulation panels for location inside the hollow walls of cooled chambers to reduce heat transfer losses therethrough.
Since heat transfer takes place by conduction, radiation and convection, an insulation panel must efficiently reduce all three modes of heat transfer. It is known that a highly effective parallel-sided insulation panel can be created, in which heavier-than-air gas filled narrow cavities are formed by a series of thin reflective sheets. For example, in U.S. Pat. No. 1,969,621 to Munters (Munters I) a hermetically sealed casing including sidewalls made of aluminum, iron, or other gas-tight substance contains a plurality of thin sheet members made of aluminum foil or the like separated by cardboard frames. The interior of the sealed panel is filled with a gas having a lower coefficient of heat conductivity than air at a corresponding pressure and temperature. Heavy gases recommended in Munters I include methylchloride (CH.sub.3 C1), dichloroldifluoromethane (CC1.sub.2 F1.sub.2), and methylbromide (CH.sub.3 Br), and the distance between adjacent partitions or foils is recommended to be less than 5 mm. to prevent convection currents in the gas. The gas may be introduced into the panel by evacuating and filling the insulation casing while the casing is wholly contained in a pressure vessel, so that the pressure inside and outside the insulation casing can be maintained the same. Endwalls are made of a material of low heat conductive capacity, e.g. celluloid or the like or a nickel-iron alloy, with the walls pasted on with a polymeric vinyl acetate.
U.S. Pat. Nos. 2,065,608 and 2,162,271, both also issued to Munters and describing a similar insulation panel, disclose that convection currents between adjacent gas layers must be prevented to obtain effective insulation across the entire assembly, and that spacing between adjacent foil members defining the cavities should be approximately 4 mm.
Conduction heat transfer through solid materials is limited to the periphery of the panel. Radiation heat transfer across the panel is minimized by the introduction of multiple reflective layers, with the effectiveness being increased by the number of reflective sheets in the assembly and by making both sides of each sheet reflective. If an enclosed gas space can be made thin enough, essentially only a conductive heat transfer mode through the thin gas layer is established. Ideally, the gas spaces should be of uniform thickness bounded by plain, smooth, parallel and preferably high reflective surfaces with no leakage of gas into or from the enclosure. Experimental evidence supports the belief that the use of heavy molecular gasses, such as those suggested in Munters I, leads to an effective insulating value of the order of 0.06 Btu-in/hr-ft.sup.2 -.degree.F. at room temperatures.
No free-convection currents occur in a fluid which is enclosed between two parallel horizontal plates so long as the temperature of the upper plate is higher than the temperature of the lower one, so that the heat transfer takes place only by conduction across the heavy gas layer.
The situation is different when a fluid is enclosed between two horizontal surfaces of which the upper surface is cooler than the lower one. Since the heat transfer now occurs from the lower toward the upper surface, the fluid between the two surfaces assumes such tempertures that the colder fluid layer is situated above the warmer one. For a gas whose density decreases with increasing temperature this leads to an unstable situation, but does not give rise to convection currents so long as the Rayleigh Number is below 1700.
For vertical fluid layers, the fluid rotates slowly at low values of Reynolds number, moving upward along the heated surface and downward along the cooled surface. At sufficiently low Rayleigh numbers, the streamlines are parallel to the vertical surfaces over the major portion of the fluid layer and are closed near the upper and lower ends. Thus the heat transfer in the central portion of the fluid is essentially by conduction only. In principle, therefore, so long as the Rayleigh number is below 1700, the heat transport across the gas-filled thin cavity becomes independent of orientation and takes place essentially by conduction across only the heavy gas. Hence insultation panels containing such cavities are effective vertically or horizontally and at the top or bottom of an insulated chamber, provided the Rayleigh Number is kept below 1700.
The Rayleigh number is defined as: EQU Ra=GrPr=C.sub.p g.beta.(T.sub.H -T.sub.c).rho..sup.2.delta.3 /k.mu.
where
Gr: Grashof number PA1 Pr: Prandtl number PA1 .rho.: fluid density (lbm/ft.sup.3) PA1 .beta.: expansion coefficient (.degree.F..sup.-1) PA1 C.sub.p : specific heat PA1 k: thermal conductivity of the fluid (Btu/hr-ft-.degree.F.) PA1 .mu.: fluid viscosity (lbm/ft-hr) PA1 T.sub.H, T.sub.C : hot and cold surface temperatures (.degree.F.) PA1 .delta.: fluid layer thickness (ft).
It is clear from the preceding that given a particular insulation problem, with specific high and low temperatures T.sub.H and T.sub.C, the designer may select a gas having suitable natural properties C.sub.p, .rho., .beta., .mu. and k and may also select the fluid layer thickness to obtain the desired essentially conductive flow through the heavy gas by ensuring a Rayleigh Number less than 1700.
Although the theory is well understood, a full realization of the expected benefits thereof requires the developement of a practical manufacturing method for producing such multi-cavity, gas-filled insulation panels.